Junction type laser diodes formed in a body of a group III-V semiconductor material have found use in optoelectronic circuits because of their small size and relatively low power requirements. In optoelectronic circuits using such laser diodes, it is important to be able to switch the laser diode on and off at high rates, and to maintain a stabilized laser diode output. The fast switching can be achieved by means of a drive circuit formed of field effect transistors (FETs). Stable laser diodes can be obtained by monitoring the output of one of the facets of the laser diode with a photodiode and using the photodiode in a feedback loop.
FIG. 1 shows a typical circuit 10 having n-channel depletion mode junction field effect transistors T1, T2, T3, T4 and T5, a laser diode LD, a photodiode PD, resistors R1 and R2 and a feedback circuit FBC. Feedback circuit FBC is coupled from an output (VOUT) terminal, which is coupled to the source of transistor T5 and to a first terminal of resistor R2, to the gate of transistor T1. Transistor T1 controls the amount of current which flows therethrough into transistors T2 or T3. Transistor T4 serves to provide a bias current into laser diode LD. The current provided by T4 to laser diode LD is not sufficient by itself to cause laser diode LD to emit light. Transistors T2 and T3 form a differential pair and current from transistor T1 will flow through the one of transistors T2 and T3 which has the gate coupled to a high voltage. The other gate of the two transistors will be at a low voltage. The a' at the gate of transistor T3 indicates that the voltage level applied thereto is the inverse of that applied to the gate (a) of transistor T2. When current from transistors T1 and T4 both flow through laser diode LD, LD emits laser light. Some of the emitted laser light is incident upon photodiode PD and modifies the voltage level at the gate of transistor T5. The current flow through transistor T5 is thus modulated by the amount of laser light incident upon the photodiode PD. Feedback circuit FBC feeds back part of the output voltage (VOUT) to the gate of transistor T1 and thus modulates the current flow through transistor T1. Accordingly, the amount of laser light emitted by laser diode LD and incident on photodiode PD modulates the current flow into the laser diode LD and thus stabilizes same. Although the circuit 10 can be made of discrete components or several integrated circuits connected together with the laser diode, it is desirable to have an optoelectronic integrated circuit which includes the laser diode, photodiode, the FETs and the resistors as a single solid state structure.